1. Field
The present disclosure relates generally to wireless communication devices and, more specifically, to a mixer in a receiver front end that provides stepped gain control.
2. Background
Receivers for wireless communication systems typically require low noise amplifiers (LNAs) followed by double-balanced mixers to pre-amplify incoming signals and to down-convert those signals to an appropriate intermediate frequency (IF) or baseband frequency. Typical receivers have a signal-to-noise ratio (SNR) of no more than about 20 dB. The signal-to-noise ratio imposes a rough limit on the data throughput that a communication system can transmit. For example, a system conforming to the IEEE 802.11g standard with a signal-to-noise ratio of about 20 dB cannot transmit much more than about 25 MB/second of data. Newer wireless communications systems are being developed that are based on orthogonal frequency-division multiplexing (OFDM) and allow higher data rates to be transmitted than is achievable with existing systems that are based on Code Division Multiple Access (CDMA) or Time Division Multiple Access (TDMA).
OFDM achieves higher spectral efficiency by removing guard bands and by spacing carrier sub-channels closely together until they actually overlap. The frequencies of the carrier sub-channels are orthogonal (perpendicular in a mathematical sense) and allow the spectrum of each sub-channel to overlap other sub-channels without interfering with them. The commercial availability of Fast Fourier transform (FFT) chips has made it feasible to demodulate each sub-channel signal. In addition, quadrature amplitude modulation is typically employed to increase the data throughput even more. These newer systems based on OFDM operate under communications standards such as: 3GPP Long-Term Evolution (LTE), Ultra Mobile Broadband (UMB) also known as Evolution-Data Optimized Revision C, WiFi (IEEE 802.11a/g), WiMax (IEEE 802.16) and Digital Video Broadcasting (DVB).
The operation of these newer OFDM communication systems, however, depends on the ability of the receiver to achieve higher signal-to-noise ratios than those of existing wireless communication systems. Moreover, in addition to having a high signal-to-noise ratio at only one particular signal level, a receiver for an OFDM system should also maintain a high signal-to-noise ratio over a large gain control range. For example, it is desirable to maintain a high signal-to-noise ratio over a gain range of at least 35 dB. The signal-to-noise ratio tends to zig-sag over a large gain range where the receiver has just a few large gain steps. Thus, a receiver with many fine gain steps can be used to achieve a consistently high and smooth signal-to-noise ratio over a large gain range.
Power consumption is also an important consideration in the design of receiver front ends, especially for receivers in handheld wireless devices. In a receiver front end, power consumption can be reduced by using passive mixers and avoiding series-configured low noise amplifiers (LNAs). Thus, a design for a receiver is sought that can achieve a smooth signal-to-noise ratio of at least 30 dB over a gain range of at least 35 dB but that maintains this signal-to-noise ratio using only passive mixers and without using series configured low noise amplifiers.